Process for producing glass substrate

ABSTRACT

The present invention relates to a process for producing a glass substrate, the process including: a lapping step of grinding a main surface of a glass substrate; and a main-surface polishing step of polishing the main surface of the glass substrate after the lapping step, in which the lapping step includes: a primary lapping step of grinding the main surface of the glass substrate with a free abrasive or a fixed abrasive; a secondary lapping step of grinding the main surface of the glass substrate with a fixed abrasive having a smaller particle size than the free abrasive or the fixed abrasive used in the primary lapping step; and a cleaning step of cleaning the main surface of the glass substrate by at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning, after the primary lapping step but before the secondary lapping step.

FIELD OF THE INVENTION

The present invention relates to a process for producing a glass substrate. More particularly, the invention relates to a process for producing a glass substrate, for example, for a magnetic disk for use in magnetic-disk devices such as hard-disk drives (hereinafter referred to as “HDDs”).

BACKGROUND OF THE INVENTION

In recent years, glass substrates are being used as one kind of magnetic-disk substrates suitable for increases in recording density. Since glass substrates have higher rigidity than metallic substrates, glass substrates are suitable for increases in the rotation speed of magnetic-disk devices. Furthermore, since a smooth and flat surface can be obtained on glass substrates, glass substrates are suitable for reducing the levitation height of magnetic heads, thereby attaining an improvement in S/N ratio and an increase in recording density.

Process for producing such a glass substrate for a magnetic disk generally include: a shape-processing step of forming the shape of the glass substrate; a lapping step constituted of a rough lapping step of grinding the main surfaces of the glass substrate and a precise lapping step of grinding the main surfaces of the glass substrate with an abrasive having a smaller particle size than the abrasive used in the rough lapping step; and a polishing step of polishing the lapped main surfaces of the glass substrate to desired surface roughness.

According to the process for producing a glass substrate for a magnetic disk which is disclosed in patent document 1, an inner rotating gear and an outer rotating gear are rotated and a slurry prepared by incorporating a free abrasive into a grinding fluid is supplied to the main surfaces of a work to thereby grind the main surfaces of the glass substrate disposed in a carrier (rough lapping step). Subsequently, a hole is cored at the center of the glass substrate, and the outer peripheral edge and inner peripheral edge are subjected to predetermined chamfering to conduct shape processing (edge shape-processing step). The outer peripheral edge and inner peripheral edge of the glass substrate are mirror-polished (edge polishing step). Subsequently, the main surfaces of the glass substrate which has undergone the edge polishing are ground with a fixed abrasive (fixed-abrasive grinding step). Thereafter, the main surfaces of the glass substrate are polished for the purpose of removing the scratches or the like and distortions which remain after the fixed-abrasive grinding step (main-surface polishing step).

The fixed-abrasive grinding step conducted in the process for producing a glass substrate for a magnetic disk disclosed in patent document 1 serves as a substitute for the precise lapping step which has been conducted hitherto, and has an advantage that a fixed abrasive having a far smaller particle size than the free abrasives used in the precise lapping step can be used and a reduction in surface removal amount can be attained in a later step. Patent document 1 discloses that in the process for producing a glass substrate for a magnetic disk, the main surfaces of the glass substrate which has undergone the edge polishing step are cleaned with water before the fixed-abrasive grinding step.

-   Patent Document 1: JP-A-2009-99250

SUMMARY OF THE INVENTION

However, in the glass substrate which has undergone the rough lapping step, the abrasives that were dispersed in the slurry used in the rough lapping step are deposited to the main surfaces of the glass substrate so that the abrasives stick in the main surfaces. Because of this, even when the main surfaces of the glass substrate are merely cleaned with water, the deposited abrasives cannot be removed. Since the fixed abrasive is constituted of a molded grindstone including a diamond abrasive, the main surfaces are apt to be affected by contaminations, and when the main surfaces of the glass substrate having the deposited abrasives are ground, large scratches generate. Thus, it has been impossible to enable the fixed abrasive to fully exhibit the performance thereof.

Accordingly, an object of the invention is to provide a process for producing a glass substrate, which enables a fixed abrasive to fully exhibit the performance thereof and which makes it possible to efficiently produce a glass substrate.

Namely, the present invention relates to the following items (1) to (5).

(1) A process for producing a glass substrate, the process comprising:

a lapping step of grinding a main surface of a glass substrate; and

a main-surface polishing step of polishing the main surface of the glass substrate after the lapping step,

wherein the lapping step comprises:

a primary lapping step of grinding the main surface of the glass substrate with a free abrasive or a fixed abrasive;

a secondary lapping step of grinding the main surface of the glass substrate with a fixed abrasive having a smaller particle size than the free abrasive or the fixed abrasive used in the primary lapping step; and

a cleaning step of cleaning the main surface of the glass substrate by at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning, after the primary lapping step but before the secondary lapping step.

(2) The process for producing a glass substrate according to (1), further comprising, after the lapping step but before the main-surface polishing step, a cleaning step of cleaning the main surface of the glass substrate by at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning.

(3) The process for producing a glass substrate according to (1) or (2), wherein the acid cleaning includes cleaning the main surface of the glass substrate with a fluorine-based solution.

(4) The process for producing a glass substrate according to any one of (1) to (3), wherein the cleaning step includes cleaning the main surface of the glass substrate by the acid cleaning, followed by cleaning with a detergent containing an alkali.

(5) The process for producing a glass substrate according to any one of (1) to (4), wherein the glass substrate is a glass substrate for a magnetic disk.

According to the invention, after the primary lapping step, the main surface of the glass substrate is cleaned by at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning, before the secondary lapping step in which grinding is conducted with a fixed abrasive. Due to this procedure, contaminations such as abrasives, that deposited to the main surface of the glass substrate in the primary lapping step and that cannot be removed by ordinary water cleaning can be removed. Thus, in the subsequent secondary lapping step for grinding with a fixed abrasive, the glass substrate can be prevented from receiving scratches due to contaminations such as abrasives and the performance of the fixed abrasive can be fully exhibited. As a result, a glass substrate can be efficiently produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a glass substrate for a magnetic disk.

FIG. 2 is a flowchart showing a process for producing a glass substrate of the invention.

FIG. 3 is a stereomicrophotograph of a main surface of a glass substrate which has undergone primary lapping.

FIG. 4 is a stereomicrophotograph of a main surface of a glass substrate which has undergone primary lapping and subsequent water cleaning.

FIG. 5 is a stereomicrophotograph of a main surface of a glass substrate which has undergone primary lapping and subsequent scrub cleaning.

FIG. 6 is a stereomicrophotograph of a main surface of a glass substrate which has undergone primary lapping and subsequent acid cleaning.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the process for producing a glass substrate of the invention is explained in detail with reference to drawings. Incidentally, the process for producing a glass substrate of the invention can be used for producing any of glass substrates for use in magnetic disks, photomasks, liquid-crystal TVs, plane lenses, etc. Here, the process is explained with respect to the production of a glass substrate to be used as the glass substrate of a magnetic disk for mounting in HDDs or the like, as an example. However, the glass substrate to be produced should not be construed as being limited to that example.

The glass substrate for a magnetic disk has an outer diameter of 15 to 30 mm, an inner diameter of 5 to 12 mm, and a plate thickness of 0.35 to 0.5 mm, and the magnetic disk may be produced as a magnetic disk having predetermined diameters, such as, for example, a “0.8-inch magnetic disk” (inner diameter, 6 mm; outer diameter, 21.6 mm; plate thickness, 0.381 mm) or a “1.0-inch magnetic disk” (inner diameter, 7 mm; outer diameter, 27.4 mm; plate thickness, 0.381 mm). The magnetic disk may also be produced as a “2.5-inch magnetic disk”, a “3.5-inch magnetic disk”, or the like. Incidentally, the term “inner diameter” means the diameter of the circular hole at the center of the glass substrate.

FIG. 1 is a perspective view showing the structure of a glass substrate for a magnetic disk. As shown in FIG. 1, the glass substrate 1 for a magnetic disk has a circular hole 2 at the center. Since this glass substrate 1 for a magnetic disk is made of a glass, excellent smoothness can be realized by mirror polishing. In addition, since the material has high hardness and high rigidity, the substrate has excellent impact resistance. Although glasses are brittle materials, the fracture strength thereof can be improved by a strengthening treatment, such as chemical strengthening or strengthening by air blast cooling, or crystallization.

Examples of the material of the glass substrate 1 for a magnetic disk include lithium silicate glass, aluminosilicate glass, lithium aluminosilicate glass, aluminoborosilicate glass, soda-lime glass and borosilicate glass. However, aluminosilicate glass is preferred. It is also possible to use an amorphous glass or a crystallized glass. In the case where the soft-magnetic layer to be formed on the glass is amorphous, it is preferred to employ an amorphous glass. For example, the glass components of an aluminosilicate glass include, in terms of % by mole, 57 to 74% of SiO₂, 0 to 2.8% of ZnO₂, 3 to 15% of Al₂O₃, 7 to 16% of LiO₂, and 4 to 14% of Na₂O. The glass components of another aluminosilicate glass include, in terms of % by mass, 50 to 65% of SiO₂, 5 to 15% of Al₂O₃, 2 to 7% of Na₂O, 4 to 9% of K₂O, 0.5 to 5% of MgO, 2 to 8% of CaO, and 1 to 6% of ZrO₂. Glass components for obtaining a high Young's modulus (100 GPa or higher) include, in terms of % by mole, 45 to 65% of SiO₂, 0 to 15% of Al₂O₃, 4 to 20% of Li₂O, 1 to 8% of Na₂O, 0 to 21% of CaO, 0 to 22% of MgO, 0 to 16% of Y₂O₃, 1 to 15% of TiO₂, and 0 to 10% of ZrO₂.

This embodiment of the process for producing a glass substrate for a magnetic disk is explained next. As shown in FIG. 2, this embodiment of the process for producing a glass substrate 1 for a magnetic disk includes:

(1) a shape-processing step (S1), (2) a chamfering step (S2), (3) a primary lapping step (S3), (4) a first cleaning step (S4), (5) a secondary lapping step (fixed-abrasive grinding step) (S5), (6) a second cleaning step (S6), (7) an edge polishing step (S7), and (8) a main-surface polishing step (S8).

In this specification, the main-surface grinding or grinding step conducted prior to the main-surface polishing step of polishing the main surface, is called lapping or a lapping step, regardless of kinds of abrasives, and this lapping step includes at least a primary lapping step (S3), a first cleaning step (S4), and a secondary lapping step (fixed-abrasive grinding step) (S5).

The shape-processing step (S1) is a step of preparing a circular glass substrate 1 for a magnetic disk. A through-hole (inner hole) is cored at the center of a rectangular plate glass, and a circular glass plate is cut out thereof.

The chamfering step (S2) is a step of chamfering the edges of the circular glass cut out of the plate glass (the edges are the line of intersection of the main surfaces and the inner peripheral edge forming the through hole and the intersecting point of the main surfaces and the outer peripheral edge). The term “main surfaces” herein means the annular portions on the front and back sides of the glass substrate 1 for a magnetic disk.

The primary lapping step (S3) is a step of grinding the upper and lower surfaces of the glass substrate 1 for a magnetic disk with a free abrasive or a fixed abrasive using a both-side processing machine. In the case of using a free abrasive, the surfaces are ground using metallic platens while supplying a slurry which contains any of an alumina abrasive, zirconia abrasive, silicon carbide abrasive, and diamond abrasive each typically having an average particle size of 8 to 40 μm and includes water as the main dispersion medium. In the case of using a fixed abrasive, a fixed-abrasive pad constituted of a sheet and, fixed thereto, a diamond abrasive typically having an average particle size of 8 to 38 μm is attached to each of the upper and lower platens of a both-side processing machine, and the main surfaces of the glass substrate 1 a for magnetic disk are ground therewith while supplying a coolant. In this embodiment, since lapping is conducted after the chamfering step (S2), the influence of chipping can be reduced.

The particle size D1 of the abrasive to be used in the primary lapping step is larger than the particle size D2 of the abrasive to be used in the secondary lapping step. For example, it is preferred that when D1 is 20 to 40 μm or 21 to 40 μm, then D2 should be 4 to 15 μm, and that when D1 is 9 to 19 μm or 9 to 20 μm, then D2 should be about 1 to 6 μm.

The first cleaning step (S4) is a cleaning step which is conducted after the primary lapping step (S3) but before the secondary lapping step (fixed-abrasive grinding) (S5) and in which the main surfaces of the glass substrate 1 for a magnetic disk are precisely cleaned. This cleaning step is conducted before the secondary lapping step (fixed-abrasive grinding) (S5), in order to remove the contaminations including deposited abrasives that were dispersed in the slurry or dropped from the fixed-abrasive pads and deposited, in the primary lapping step (S3), to the main surfaces of the glass substrate so as to stick therein, and further including glass debris. Specifically, at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning is conducted. The ultrasonic cleaning and scrub cleaning may be ones using water as a medium. Alternatively, the ultrasonic cleaning or scrub cleaning may be performed together with the acid cleaning using an acid. As a result, the abrasives deposited, in the primary lapping step, to the main surfaces of the glass substrate so as to stick therein and were unable to be removed by ordinary cleaning with water, and other contaminations can be removed. Compared to cleaning with water, acid cleaning is more effective in removing contaminations since the main surfaces of the glass substrate 1 for a magnetic disk is dissolved.

In the ultrasonic cleaning, contaminations such as abrasives deposited to the main surfaces of the glass substrate 1 for a magnetic disk are removed by utilizing the cavitation or acceleration occurring in the cleaning liquid by the action of ultrasonic vibrations and further utilizing a straight flow which is the flow of the fluid (cleaning liquid). A known machine is used to conduct the ultrasonic cleaning.

The scrub cleaning is a technique in which porous polymer rolls called scrubbing rolls are brought into contact with the main surfaces of the glass substrate 1 for a magnetic disk, while being rotated, to remove contaminations deposited to the main surfaces of the glass substrate 1 for a magnetic disk, thereby cleaning the surfaces. This cleaning can be conducted using a known machine. Besides the technique described above, use may be made of a method in which a sponge or the like is manually rubbed against each main surface of the glass substrate 1 for a magnetic disk to thereby clean the surface.

The acid cleaning is a technique in which the glass substrate 1 for a magnetic disk is cleaned by immersing the substrate 1 in an acid, such as sulfuric acid, phosphoric acid, nitric acid, hydrofluoric acid, or hydrochloric acid, a mixture of two or more of these acids, or a detergent prepared by adding a salt of any of these acids (e.g., ammonium fluoride or potassium nitrate) to any of these acids. Of these, it is preferred to treat with a fluorine-based solution. It is more preferred to treat with a fluorine-based solution having a pH of 2 to 7. By using this solution, portions having low acid resistance can be inhibited from being selectively dissolved away.

It is preferred that the glass substrate 1 should be treated with a fluorine-based solution having a pH of 2 to 7 and then treated with a detergent containing an alkali. Thus, high cleanliness of the main surfaces of the glass substrate 1 for a magnetic disk can be obtained.

Namely, in glasses constituted of multiple components, the components having low resistance to acids are alkali metal oxides, alkaline earth metal oxides, aluminum oxide, and the like, while the components having high resistance to acids are silica oxide, titania oxide, zirconia oxide, and the like. However, when such glasses are treated with a detergent including a fluorine-based solution having a pH of 2 to 7, the components having low resistance to acids can be inhibited from being selectively dissolved. Cleaning with water may be conducted before and/or after the treatment with a detergent containing an alkali.

The fluorine-based solution is preferably one obtained by adding a pH adjuster to a solution of hydrofluoric acid to adjust the pH of the solution to 2 to 7. From the standpoint of producing a buffering effect in the solution of hydrofluoric acid, it is preferred that the pH adjuster should be one or more fluorides, specifically, at least one member selected from sodium fluoride, potassium fluoride, ammonium fluoride, ammonium borofluoride, and ammonium silicofluoride.

From the standpoint of etching the glass, it is preferred that the fluorine-based solution to be used should be a solution of ammonium hydrogen fluoride and/or a solution of hydrosilicofluoric acid.

When a pH adjuster is added to the fluorine-based solution in a concentration not lower than a certain value, the solution becomes substantially neutral, with the pH thereof being 7. Consequently, a preferred range of the pH of the fluorine-based solution for inhibiting selective dissolution is 2 to 7. The reason why the pH of the fluorine-based solution is limited to 2 or higher is as follows. In case where the pH of the solution is lower than 2, the pH adjuster does not substantially function and selective dissolution of components having low acid resistance occurs readily.

The alkaline detergent to be used is an aqueous solution usually containing an alkali ingredient, a surfactant, and a chelating agent as main components. For example, the detergent to be used can be selected from commercial alkali detergents.

Examples of the alkali ingredient include tetramethylammonium hydroxide (hereinafter referred to as TMAH), caustic soda, potassium hydroxide, sodium carbonate, potassium carbonate, and ammonia. Whichever alkali ingredient is used, electrostatic repulsion increases as the pH of the solution used increases. It is therefore preferred, from the standpoint of obtaining satisfactory cleanliness, that the alkaline detergent to be used should be adjusted so as to have a higher pH.

Of those alkali ingredients, TMAH has an advantage that since this compound has a structure in which the hydrogen moieties of an ammonium ion have been replaced with methyl groups, which are bulky, the factor of steric hindrance which is encountered when this alkali ingredient attacks the glass substrate is large and the substrate is less apt to be damaged.

Thus, when TMAH is employed, the alkaline detergent can be used at a higher pH and even better cleanliness can be obtained as compared with cases where alkali ingredients other than TMAH are used. Consequently, it is more preferred to use TMAH as the alkali ingredient.

The surfactant is not particularly limited. For example, use may be made of: a nonionic surfactant such as a polyoxyethylene alkyl ether or a polyoxyethylene derivative; a quaternary ammonium salt such as lauryltrimethylammonium chloride; a higher amine/halogen acid salt such as a hardened beef tallow amine; a cationic surfactant, e.g., an alkylpyridinium halide such as dodecylpyridinium chloride; an anionic surfactant, e.g., a sodium alkyl sulfate, fatty acid sodium salt, or alkylarylsulfonic acid salt; or an amphoteric surfactant, e.g., an amino acid salt such as sodium laurylaminopropionate.

The chelating agent is not particularly limited. For example, use may be made of dimethylglyoxime, dithizone, oxine, acetylacetone, glycine, ethylenediaminetetraacetic acid, nitrilotriacetic acid, or the like.

The surfactant concentration and chelating agent concentration of the alkaline detergent are not particularly limited. However, the alkaline detergent to be used, for example, has a surfactant concentration of 0 to 1 wt % and a chelating agent concentration of 0 to 1 wt %.

In the secondary lapping step (fixed-abrasive grinding step) (S5), the upper and the lower surfaces of the glass substrate 1 for a magnetic disk are ground with a fixed abrasive using a both-side processing machine. As stated above, the fixed abrasive to be used is a diamond abrasive having a particle size smaller than the average particle size of the free abrasive or the fixed abrasive used in the primary lapping step (typically, the diamond abrasive has a particle size of 1 to 20 μm). A fixed-abrasive pad constituted of a sheet and the diamond abrasive fixed thereto is attached to each of the upper and lower platens of the both-side processing machine, and the main surfaces of the glass substrate 1 for a magnetic disk are ground therewith while supplying a coolant. Thus, smaller particles than in conventional precise lapping can be used, and a glass substrate 1 for a magnetic disk which has desired surface roughness can be efficiently obtained. In the secondary lapping step (fixed-abrasive grinding step) (S5), the main surfaces of the glass substrate 1 for a magnetic disk are apt to be influenced by contaminations. However, since the precise cleaning described above was conducted prior to the secondary lapping step (fixed-abrasive grinding step) (S5), the main surfaces of the glass substrate 1 for a magnetic disk can be inhibited from receiving scratches.

The second cleaning step (S6) is a cleaning step in which the main surfaces of the glass substrate 1 for a magnetic disk are precisely cleaned after the lapping step, i.e., after the secondary lapping step (fixed-abrasive grinding step) (S5), but before the main-surface polishing step (S8). This second cleaning step (S6) is conducted in order to remove the glass debris which generated in the secondary lapping step (fixed-abrasive grinding step) (S5) and the diamond abrasives dropped from the fixed-abrasive pads. Specifically, at least one selected from the group consisting of ultrasonic cleaning using water, scrub cleaning and acid cleaning is conducted as in the first cleaning step (S4). An explanation on details thereof is omitted because the same treatment as in the first cleaning step (S4) is performed in the second cleaning step (S6).

The edge polishing step (S7) is a step of mirror-polishing the inner peripheral edge and outer peripheral edge of the glass substrate 1 for a magnetic disk which were chamfered.

The main-surface polishing step (S8) is a step of polishing the main surfaces of the glass substrate 1 for a magnetic disk while supplying a ceria slurry. This main-surface polishing step (S8) is conducted in order to eliminate fine recesses and protrusions which were formed on the main surfaces of the glass substrate 1 for a magnetic disk in the secondary lapping step (fixed-abrasive grinding step) (S5) and to thereby obtain mirror-polished main surfaces. More specifically, the main-surface polishing step (S8) is, for example, conducted in two stages in the following manner. In the first main-surface polishing step, a ceria slurry prepared by dispersing a ceria abrasive typically having an average particle size of 0.1 to 5.0 μm in water is supplied, and a both-side processing machine in which a rigid urethane foam pad or a sueded urethane pad has been attached to each of the upper and lower platens is used to mirror-polish the glass substrate 1 for a magnetic disk. Subsequently, ultrasonic cleaning is conducted using water and a detergent, strong acid, or strong alkali. In the subsequent second main-surface polishing step, a silica slurry prepared by dispersing a colloidal silica abrasive typically having an average particle size of 0.005 to 0.2 μm in water is supplied, and a both-side processing machine in which a sueded urethane pad has been attached to each of the upper and lower platens is used to polish the glass substrate 1 for a magnetic disk and make the surface of the glass substrate smoother. Incidentally, the main-surface polishing step need not be always conducted in two stages, and the step may be conducted in three stages in order to mirror-polish the main surfaces of the glass substrate 1 for a magnetic disk to a higher degree.

In this embodiment of the process for producing a glass substrate, the order of steps can be suitably changed so long as the process includes the first cleaning step (S4), in which precise cleaning is performed as described above, after the primary lapping step (S3) but before the secondary lapping step (fixed-abrasive grinding step) (S5), in which grinding is conducted with a fixed abrasive. For example, with respect to the chamfering step (S2) and the primary lapping step (S3), the order may be reversed so that the edges of the circular glass substrate 1 for a magnetic disk are chamfered after the glass substrate 1 has undergone the primary lapping. As a result, the front surface and back surface of the glass substrate 1 for a magnetic disk can be inhibited from having a difference in height.

Furthermore, the process may include steps other than those shown above. For example, the process may include a chemical strengthening step in which the glass substrate is chemically strengthened after the main-surface polishing step (S8). In the chemical strengthening step, ions (e.g., Li⁺ and Na⁺ in the case where aluminosilicate glass was used) present in the surfaces of the glass substrate (for example, in a surface layer ranging to a depth of about 5 μm from the glass substrate surfaces) are exchanged for ions (Na⁺ and K⁺) having a larger ionic radius than those ions to thereby impart compressive stress to the glass surfaces. As a result, the rigidity of the glass substrate can be enhanced.

In this embodiment, both surfaces of a glass substrate 1 for a magnetic disk were simultaneously processed using a both-side processing machine. However, the process is not limited thereto, and the two surfaces may be separately processed successively.

Example

Example of the invention is explained below.

As a glass substrate, a 2.5-inch glass substrate for a magnetic disk was used. This glass substrate was subjected to 15-minute primary lapping using a both-side processing machine while supplying a slurry containing an alumina abrasive with an average particle size of 5 to 20 μm and containing water as the main dispersion medium. After this primary lapping step, the main surfaces of the glass substrate were cleaned by different methods before a secondary lapping step. The main surfaces thus cleaned were examined through stereomicrophotographs.

FIG. 3 is a stereomicrophotograph of a main surface of the glass substrate which underwent the primary lapping. The areas which appear to be white streaks are contaminations including abrasives and glass debris that have deposited to the main surface of the glass substrate as a result of the primary lapping.

FIG. 4 is a stereomicrophotograph of a main surface of the glass substrate in which, after the primary lapping, the main surface was cleaned with water for 3 minutes. Although the contaminations which appear to be white streaks have decreased from the state observed after the primary lapping shown in FIG. 3, it can be seen that contaminations still remain. It is presumed therefrom that the contaminations such as abrasives and glass debris, which deposited in the first lapping are present so as to stick in the main surface of the glass substrate.

FIG. 5 is a stereomicrophotograph of a main surface of the glass substrate in which, after the primary lapping, the main surface was scrubbed for 3 minutes by rubbing the main surface with a sponge by hand using water as a cleaning liquid. It can be seen that the contaminations which appeared to be white streaks in FIG. 3 have substantially disappeared. It was thus ascertained that scrub cleaning is effective against the contaminations such as abrasives and glass debris, which deposited in the primary lapping. Besides the scrub cleaning, ultrasonic cleaning was ascertained to produce the same effect.

FIG. 6 is a stereomicrophotograph of a main surface of the glass substrate in which, after the primary lapping, the main surface was acid-cleaned by immersing the main surface thereof for 1 minute in a solution of ammonium hydrogen fluoride having a pH of 2.8. It can be seen that the contaminations which appeared to be white streaks in FIG. 3 have substantially disappeared. It was thus ascertained that acid cleaning also is effective against the contaminations such as abrasives and glass debris, which deposited in the primary lapping.

In this Example, the main surfaces of a glass substrate which had undergone a primary lapping step were cleaned by different methods before a secondary lapping step and examined. However, the same results are obtained when the main surfaces of the glass substrate which has undergone a secondary lapping step are cleaned by ultrasonic cleaning, scrub cleaning, or acid cleaning before a main-surface polishing step.

As explained above, according to the process for producing a glass substrate of the invention, the main surfaces of the glass substrate are cleaned by at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning after the primary lapping step but before the secondary lapping step (fixed-abrasive grinding step). Due to this procedure, contaminations such as abrasives that deposited to the main surfaces of the glass substrate in the primary lapping step can be removed. Thus, in the subsequent secondary lapping step in which grinding is conducted using a fixed abrasive (fixed-abrasive grinding step), the glass substrate can be prevented from receiving scratches due to contaminations, e.g., abrasives that cannot be removed by ordinary water cleaning, and the performance of the fixed abrasive can be fully exhibited.

Furthermore, in the secondary lapping step (fixed-abrasive grinding step), smaller particles than in conventional precise lapping can be used and, hence, a glass substrate having desired surface roughness can be efficiently obtained. Incidentally, by using a fixed abrasive to conduct grinding in the primary lapping step, the grinding can be carried out in a shorter time period.

In addition, in case where the main surfaces of the glass substrate are cleaned by at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning after the lapping step, i.e., after the secondary lapping step (fixed-abrasive grinding step), but before the main-surface polishing step, the glass debris which generated in the secondary lapping step (fixed-abrasive grinding step) and the diamond abrasives which dropped from the fixed-abrasive pads can be removed. Thus, the glass substrate can be prevented from receiving scratches due to contaminations in the main-surface polishing step, and a glass substrate of high quality can be obtained in a short time period.

The invention should not be construed as being limited, in any way, to the embodiment described above, and can be carried out in various modes without departing from the spirit of the invention.

While the invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Incidentally, the present application is based on Japanese Patent Applications No. 2009-289677 filed on Dec. 21, 2009, and the contents are incorporated herein by reference.

Also, all the references cited herein are incorporated as a whole.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   1 Glass substrate for magnetic disk (glass substrate)     -   2 Circular hole 

1. A process for producing a glass substrate, said process comprising: a lapping step of grinding a main surface of a glass substrate; and a main-surface polishing step of polishing the main surface of the glass substrate after the lapping step, wherein the lapping step comprises: a primary lapping step of grinding the main surface of the glass substrate with a free abrasive or a fixed abrasive; a secondary lapping step of grinding the main surface of the glass substrate with a fixed abrasive having a smaller particle size than the free abrasive or the fixed abrasive used in the primary lapping step; and a cleaning step of cleaning the main surface of the glass substrate by at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning, after the primary lapping step but before the secondary lapping step.
 2. The process for producing a glass substrate according to claim 1, further comprising, after the lapping step but before the main-surface polishing step, a cleaning step of cleaning the main surface of the glass substrate by at least one selected from the group consisting of ultrasonic cleaning, scrub cleaning and acid cleaning.
 3. The process for producing a glass substrate according to claim 1, wherein the acid cleaning includes cleaning the main surface of the glass substrate with a fluorine-based solution.
 4. The process for producing a glass substrate according to claim 1, wherein the cleaning step includes cleaning the main surface of the glass substrate by the acid cleaning, followed by cleaning with a detergent containing an alkali.
 5. The process for producing a glass substrate according to claim 1, wherein the glass substrate is a glass substrate for a magnetic disk. 